Light emitting diode (LED) lighting fixtures are useful for a variety of applications. Examples of these applications include commercial and household interior lighting, exterior and egress lighting, emergency lighting, automotive lighting and instrument lighting. LED lighting fixtures promise to provide improvements over existing lighting technologies. One significant improvement is a longer operating life. This improvement, however, raises different challenges than other lighting technology. Whereas other sources of light, such as fluorescent and incandescent, have a failure mode of relatively sudden and complete shut-off, LEDs typically continue to fade over time without shutting off completely. This tendency has been recognized by the solid-state lighting industry in the form of lifetime measurement criteria that defines LED life as “L70”, the hours of service at which the LED will output 70% of its original and/or rated output at the original/rated power draw.
This failure mode can be perceived as a benefit, for example in the case of an LED lamp used to light an entrance over a dark set of stairs. An incandescent light will fail immediately when the filament burns out, leaving the entrance and stairs in the dark, while an LED lamp will typically continue to provide light at some level for a much longer period of time. However, the gradual dimming can be problematic. For example, at a given power draw, less light is output, resulting in an area that is less lit than intended. This can result in safety or security problems. More lights must be turned on or the lamps must be “undimmed” in order for the light levels to reach the level desired. The result is increased power consumption. Egress lighting requirements may require lighting designers to over-specify the amount of light in an area to ensure the required emergency egress light level is available in emergency evacuations over the lifetime of a building. This can cause regulatory barriers to the acceptance and implementation of LEDs into commercial and institutional buildings. In addition, due to the difficulty, time and expense of accurately determining when an LED light has permanently dropped below the acceptable light output limit, consumers may automatically replace LEDs upon expiration of the warranty. This may result in years of useful operational life being unnecessarily wasted, reducing the cost effectiveness of using LED lighting in the first place.